1. Field of the Invention
The invention relates in general to the reduction of cogging torque of permanent magnet motors, and particularly to brushless permanent magnet motors.
2. Description of the Related Art
Permanent magnet motors include a stator core, which is typically made of a stack of thin, metal laminations. The laminations are usually round, with a central opening. The stator core thus is generally cylindrical in shape, with a cavity extending lengthwise about the central axis of the core. In brushless permanent magnet motors, each stator lamination includes radially-extending slot openings, or notches, from the central opening that are aligned when stacked to receive stator windings, or conductors. The stator core surrounds a rotor, typically consisting of a circular steel shaft with a number of permanent magnets fixed around the circumference of the shaft. However, the rotor can also comprise a stack of laminations instead of a solid steel shaft.
In permanent magnet motors, cogging torque is caused by the combination of two factors, the permanent magnet magneto-motive force and the variation of the air gap permeance between the stator and the rotor. Cogging torque is represented by the following formula:
Tcog (xcex8r)=xe2x88x92dW/dxcex8r=xe2x88x92(xc2xd) (MMF)2 (dxcex/dxcex8r), wherein
Tcog is the cogging torque;
W is the total energy of the field;
xcex8r is the rotor position angle;
MMF is the magnetic excitation of the permanent magnets; and
xcex is the air gap permeance.
In the design of permanent magnet machines, cogging torque is a concern because it adds unwanted harmonic components to the torque-angle curve, resulting in torque pulsation upon operation of the machine. Although net cogging torque is zero, it causes noise, power losses and inaccuracies, particularly in servo-positioning drives. Thus, reduction of the momentary cogging torque is desirable.
One means disclosed by the prior art to reduce cogging torque is shown in FIG. 8. FIG. 8 is a partial view of a rotor 500 on which a permanent magnet 502 is mounted in the air gap 503. The rotor 500 is surrounded by a stator 504 having a number of stator slot openings 506 forming stator teeth 508. The magnets 502 of the rotor 500 are separated by a width, represented by Ym, while the pitch of the magnets 502 is represented by xcfx84r. By varying either the magnet shape or the ratio of Ym/xcfx84r, or both, the designer can adjust the magnetic excitation of the magnets 502, and hence the resultant cogging torque.
Another method to reduce cogging torque is to reduce the variation in air gap permeance by, for example, reducing the width of the stator slot openings 506. However, stator slot openings 506 are restricted in how small they may get. If the openings 506 are too small, insertion of the stator windings is difficult. Due to this limitation, another approach to cogging torque reduction developed, adding notches to the tooth of the stator as shown in FIG. 9. FIG. 9 shows a rotor 500 with magnets 502 surrounded by stator slot openings 506 and a stator tooth 508. The stator tooth 508 has two notches 510. By manipulating the number, width and position of these notches 510, the air gap permeance is modified to reduce cogging torque. However, this approach is vulnerable to excessive saturation of the motor core caused by reduction of the flux path area on the tooth 508 resulting from, for example, paths 512. This saturation results in the reduction of torque linearity within the operating range of the motor. Further, this approach reduces efficiency and torque density as a result of the increase of the effective air gap 503. Thus, it is desired to create a machine design that reduces cogging torque, without the drawbacks of present methods.
The present invention is a stator tooth for a stator core of a permanent magnet motor, including a stator lamination forming part of a stator core, comprising a first edge formed by a first stator slot opening and extending a first radial distance from an outside edge of the stator core; a second edge formed by a second stator slot opening and extending a second radial distance from the outside edge of the stator core; and a tooth surface extending from an end of the first edge to an end of the second edge, the tooth surface including one of a single essentially straight surface segment and a plurality of surface segments, wherein each surface segment of the plurality of surface segments joins an adjacent surface segment of the plurality of surface segments at an obtuse angle. Preferably, the first radial distance is equal to the second radial distance.
In the aspect where the tooth surface includes a plurality of surface segments, the obtuse angle can be an angle between 120xc2x0 and 170xc2x0. More preferably, the obtuse angle is an angle between 160xc2x0 and 170xc2x0.
The plurality of surface segments can comprise a plurality of planar surface segments. Alternatively, the plurality of surface segments can comprise a plurality of arcuate surface segments.
In another aspect of the invention, each of the plurality of surface segments comprises a starting edge and an ending edge. The starting edge of a surface segment lies along a first circumferential path a predetermined radial distance from the outside edge of the stator core and the ending edge of the surface segment lies along a second circumferential path one of a closer predetermined radial distance from the outside edge of the stator core and a further predetermined radial distance from the outside edge of the stator core. In the preferred design of this aspect, the first circumferential path and the second circumferential path are concentric to the outside edge of the stator core.
In one aspect, the tooth surface comprises at least four surface segments. In one variation of this aspect, each of the at least four surface segments comprises a starting edge and an ending edge; and wherein the starting edge of each surface segment lies along a first circumferential path a first predetermined radial distance from the outside edge of the stator core and the ending edge of each surface segment lies along a second circumferential path one of a closer predetermined radial distance from the outside edge of the stator core than the first predetermined radial distance and a further predetermined radial distance from the outside edge of the stator core than the first predetermined radial distance. In one aspect, the at least four surface segments are four surface segments comprising a first surface segment, a second surface segment, a third surface segment and a fourth surface segment, and an obtuse angle between the first surface segment and the second surface segment is equal to an obtuse angle between the third surface segment and the fourth surface segment. In an alternative aspect, the at least four surface segments are six surface segments comprising a first surface segment, a second surface segment, a third surface segment, a fourth surface segment, and fifth surface segment and a sixth surface segment, and an obtuse angle between the first surface segment and the second surface segment is equal to an obtuse angle between the fifth surface segment and the sixth surface segment and an obtuse angle between the second surface segment and the third surface segment is equal to an obtuse angle between the fourth surface segment and the fifth surface segment.
In another variation of the aspect where the tooth surface comprises at least four surface segments, the at least four surface segments are four surface segments comprising a first surface segment, a second surface segment, a third surface segment and a fourth surface segment, and an obtuse angle between the first surface segment and the second surface segment is equal to an obtuse angle between the third surface segment and the fourth surface segment. Alternatively, the at least four surface segments are six surface segments comprising a first surface segment, a second surface segment, a third surface segment, a fourth surface segment, and fifth surface segment and a sixth surface segment, and an obtuse angle between the first surface segment and the second surface segment is equal to an obtuse angle between the fifth surface segment and the sixth surface segment and an obtuse angle between the second surface segment and the third surface segment is equal to an obtuse angle between the fourth surface segment and the fifth surface segment.
In yet another variation of the aspect where the tooth surface comprises at least four surface segments, each of the at least four surface segments comprises a starting edge and an ending edge, and the starting edge and the ending edge of up to two non-adjacent surface segments of the at least four surface segments lie along a common circumferential path a predetermined radial distance from the outside edge of the stator core. Alternatively, the at least four surface segments are six surface segments; and wherein each of the six surface segments comprises a starting edge and an ending edge; and wherein the starting edge and the ending edge of up to three non-adjacent surface segments of the six surface segments lie along a common circumferential path a predetermined radial distance from the outside edge of the stator core.
In preferred aspects of the invention, a surface area of each surface segment of the plurality of surface segments is equal.
The stator tooth for a stator core of a permanent magnet motor can also comprise a first edge formed by a first stator slot opening and extending a first radial distance from an outside edge of the stator core; a second edge formed by a second stator slot opening and extending a second radial distance from the outside edge of the stator core; and a tooth surface extending from an end of the first edge to an end of the second edge, the tooth surface including a plurality of surface segments, wherein each surface segment of the plurality of surface segments joins an adjacent surface segment of the plurality of surface segments at an obtuse angle.
In another aspect of the invention, the first radial distance is equal to the second radial distance, and a surface area of each of the plurality of surface segments is equal. In this aspect, each of the plurality of surface segments can comprise a starting edge and an ending edge wherein the starting edge of a surface segment lies along a first circumferential path a predetermined radial distance from the outside edge of the stator core and the ending edge of the surface segment lies along a second circumferential path one of a closer predetermined radial distance from the outside edge of the stator core and a further predetermined radial distance from the outside edge of the stator core.
The stator tooth of the present invention effectively reduces cogging torque without significantly increasing the effective air gap or adversely affecting the flux paths of a motor.